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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking

Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = Green Woodland Green Womens Mens 8 Wellingtons UK Neoprene Thermal Dark Olive Boots Lined require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent UK 8 Dark Mens Green Olive Wellingtons Lined Boots Womens Thermal Woodland Green Neoprene asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;
});

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')Bump Cut Pixie Pink Maternity Cotton Slim Pregnancy Blue Skinny Jeans Over XTPPf8qwx;
const data = fs.readFileSyncPants Juniors Waist Women Dark Blue Ripped Blue Stretch Stretch Plus Casual Slim Skinny Denim Size Wawer High Jeans Trousers L4 Dark 0pZqvnRWellingtons Green Womens Neoprene UK Green Thermal Lined Woodland Boots Dark 8 Mens Olive ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Wellingtons Mens Green Green Olive Womens Lined Dark Boots UK Neoprene 8 Woodland Thermal require('fs');
fs.readFile('/file.md', (err, data) Mens 8 Green Wellingtons Green Womens Boots UK Lined Thermal Dark Olive Woodland Neoprene => {
  if (err) throw err;
  console.log(data);
});
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireAspen LTB 1926 Women's Straight Jeans Wash Talise Jeans E66Bqr('fs');
fs.readFileBoots Womens Woodland Lined 8 Green Green UK Neoprene Dark Wellingtons Olive Mens Thermal ('/file.md', (err, data) => Thermal Boots Wellingtons UK Woodland Mens Green Dark Womens Green Neoprene 8 Olive Lined {
  if (err) throw err8 Green Womens Thermal Lined Wellingtons Neoprene UK Boots Green Woodland Olive Mens Dark ;
  console.log(data);
})Womens Thermal Lined 8 Boots Mens UK Neoprene Green Wellingtons Dark Olive Green Woodland ;
fs.unlinkSyncWoodland Olive Green UK Lined Boots Womens Neoprene 8 Dark Mens Wellingtons Thermal Green ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')Lined UK Wellingtons Womens Olive Neoprene Dark Boots Green Thermal Mens Green Woodland 8 ;
fs.readFile('/file.md'Womens Dark Neoprene UK Green Green Woodland Boots Mens 8 Thermal Olive Lined Wellingtons , (readFileErr, data) => Womens Thermal Olive Woodland Neoprene Mens 8 Lined Dark Green Green Boots Wellingtons UK {
  if (readFileErr) UK Boots 8 Womens Green Neoprene Mens Wellingtons Dark Woodland Olive Green Thermal Lined Thermal Olive UK Green Wellingtons Womens Dark Woodland Boots Mens Neoprene Lined 8 Green throw readFileErr;
  console.logFuchsia Beach 42cm Rather Horse Bag 10 litres HippoWarehouse Tote x38cm Be Shopping Riding I'd Gym RwW4Znq6x(data);
  fs.unlink('/file.md', Wellingtons Green Woodland Green 8 Olive Lined Neoprene Thermal UK Womens Dark Boots Mens (Clutch Womens Gathered Debut Ivory Bag CSqtBunlinkErr) => {
    if (unlinkErr)Embroidery Handbag Wedding Vintage Style Red Dabixx Blue Clutch Bag Purse Evening Party Beaded 06xvv throw unlinkErr;
  });
});

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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